Hydrocarbon combustion in diesel engines, stationary gas turbines, and other systems generates exhaust gas that must be treated to remove nitrogen oxides (NOx), which consists of NO (nitric oxide), NO2 (nitrogen dioxide), and N2O (nitrous oxide). The exhaust generated in lean-burn engines is generally oxidative, and the NOx needs to be reduced selectively with a catalyst and a reductant, which is typically ammonia or a short-chain hydrocarbon. The process, known as selective catalytic reduction (SCR), has been extensively investigated.
Power plants often utilize fossil fuels, such as coal, oil or natural gas, as the energy source. Coal and oil contain various amounts of sulfur. Treatment of exhaust from these plants using SCR demands maintaining a high NOx reduction efficiency while minimizing SO2 oxidation. Many SCR catalysts are effective in converting NOx to nitrogen and water in the presence of ammonia. However, an undesirable side reaction, the oxidation of SO2 to SO3, commonly occurs along with NOx reduction. The formation of sulfur trioxide (SO3), a component of acid rain, needs to be controlled. Thus, a catalyst that maximizes the rate of NOx reduction (kNOx) while minimizing the rate of SO2 oxidation (kSOx) is highly desirable.
SCR catalysts commonly include vanadium oxides combined with molybdenum or tungsten oxides and supported on titania. The literature includes many examples of such catalysts. In some cases, a catalyst mixture is applied to a substrate by a dip coating process (see, e.g., U.S. Pat. Nos. 5,166,122 and 7,842,644). A wet paste containing the catalyst can be rolled onto the substrate, which is then dried and calcined (see, e.g., U.S. Pat. Nos. 5,225,390; 5,045,516; 6,054,408; and 7,842,644). Alternatively, the wet paste is pressed, dried, and calcined to form a thin plate (see, e.g., U.S. Pat. Nos. 5,409,681; 5,792,432; 6,063,342; 6,710,013; and 6,759,565). Spray or dip-coating is generally not used commercially to produce articles comprising VOx-based SCR catalysts.
The industry would benefit from ways to make improved articles useful for SCR catalysis. In particular, articles having the ability to effectively reduce NOx while simultaneously controlling the rate at which sulfur dioxide is oxidized to sulfur trioxide would be valuable. Thus, articles that can yield higher kNOx/kSOx ratios in an SCR process are needed. Ideally, the articles could make more efficient use of the catalytic materials that make up their active components.